An international team of paleontologists, spearheaded by Maria McNamara of Ireland's University College Dublin, made the find while studying the remains of more than a hundred ancient frogs and salamanders.

The discovery suggests that many other fossil bones may contain well-preserved remnants of bone marrow, the scientists say.

Although fossil remains of muscles, skin, and internal organs have been found, they are rare because soft tissues usually decay before they can be fossilized.

And when traces of such tissues are found, the original organic matter has usually been replaced by minerals during fossilization.

Not so with the Spanish amphibians.

"The marrow is organically preserved," McNamara said. "The original color of the marrow is preserved."

Like modern frogs, she says, the bones show an inner zone of yellow, fatty marrow, encircled by an outer zone of red marrow.

The find will allow "incredible insights" into the makeup of ancient animals, McNamara predicts.

The simple discovery that the marrow was red likely means that the animals made red blood cells in their bones, rather than solely in their spleens, as is the case with some modern salamanders, she says.

The team announces its discovery in the August issue of the journal Geology.